The present invention relates generally to magnetic resonance (MR) imaging systems and methods. More particularly, the present invention relates to a MR imaging system equipped for real-time imaging and which permits an operator to identify and track movement of a freely moving structure of interest positioned therein.
When an object such as human tissue is subjected to an uniform magnetic field (polarizing field B.sub.0, referred to as the z direction in x, y, z coordinates), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it at their characteristic Larmor frequency. If the object, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z may be rotated, or "tipped" at a certain tipping angle, into the x-y plane to produce a net traverse magnetic moment M. A signal is emitted by the excited spins after the excitation signal B.sub.1 is terminated and this signal may be received and processed to form a MR image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x, G.sub.y, and G.sub.z) are employed. Typically, the object, or tissue, to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received nuclear magnetic resonance (NMR) signals, also referred to as MR signals, are digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
Presently, when imaging a static or quasi-static object such as the heart, because its movements are predictable or minimal, it is possible to observe the movement and identify the orientation of the object through a series of single images. Each image would be adequately presented by providing the scanning or imaging plane orientation relative to a fixed reference frame in the MR imaging system, or preferably, relative to the object itself. This scheme also works well for objects having gross bulk movements as long as the movements are known and predictable, such as in kinematic joint studies.
In contrast, with a freely moving object such as a fetus, it is often difficult to identify and track the movement and orientation of such an object from a series of single images. In fact, because the object's movements are unpredictable as well as possibly being gross bulk movements, the operator is essentially chasing and trying to keep up with the freely moving object throughout the series of images. Thus, the scanning and viewing protocol presently used for static or quasi-static objects are not well-suited for freely moving objects. First, from the resulting changes in a single image (from the preceding images), movement of a freely moving object is difficult to identify. Specifically, movement of the object in the plane of the image is readily ascertainable, but movement perpendicular to the plane of the image or out-of-plane rotations are not easy to identify from a single image.
Second, providing the imaging plane orientation of each image relative to a fixed reference frame is not useful for a freely moving object because the operator still has no information as to the orientation of the object relative to that fixed reference frame. In other words, when a patient lies down in the MR imaging system and stays in one orientation, the orientation of the patient relative to the system is known. This enables the system to present the orientation of the imaging plane with respect to the patient which is sufficient to orient the operator on what he or she is viewing. In this way, even images that are highly symmetrical, such as axial slices through the brain, have their orientation clearly identified. However, when the object is a fetus, for example, providing orientation of the imaging plane relative to the mother gives no information as to the orientation of the freely moving fetus itself. Therefore, the common practice of providing orientation information based on a fixed reference frame is insufficient when imaging freely moving objects.
Third, in many MR imaging systems, the quickest and easiest way to prescribe the desired imaging plane is by specifying a line on a scout image through which the subsequent imaging plane of the subsequent image will intersect perpendicularly. However, because these scout images are typically not updated continuously in real-time, imaging plane prescription using a scout image for a freely moving object would always be outdated and consequently result in a subsequent image which bears little resemblance to the desired image.
Thus, there is a need for a MR imaging system having one or more of the following features: the capability to image static, quasi-static, and freely moving objects; provide an improved object movement identification scheme, an improved object orientation identification scheme, and an improved imaging plane prescription scheme; provide improved archival of images such that desired images may be easily retrieved based on specific retrieval criteria; and provide improved performance parameters described above without unduly lengthening the acquisition, reconstruction, or display time of images, requiring extensive additional system components, requiring extensive operator training, nor causing significant degradation in existing image quality.